Silicon nitride is a dielectric material that is very frequently used in many applications in the manufacture of semiconductor devices. A film of silicon nitride is typically formed over a semiconductor substrate upon which semiconductor devices are being fabricated. The film is then patterned using an etching process that includes phosphoric acid. The wet chemical etching of silicon nitride has traditionally been done using hot phosphoric acid (H3PO4) at temperatures of around 160° C. The basic chemical reactions that model the etching of silicon nitride with phosphoric acid are:

The silicon nitride etch process is heavily influenced by process parameters including H3PO4 and silicon concentration, as evidenced by silica, SiO2 precipitation, temperature of the etch bath and the bath life of the hot phosphoric bath. The oxide (silicon dioxide, SiO2) etch rate is also affected by process conditions, in particular the silicon concentration in the etching bath. The oxide etch rate becomes dramatically lower as the Si concentration in the bath increases. The Si concentration therefore directly affects the silicon nitride:silicon oxide etch selectivity. It is therefore desirable to maintain the bath conditions such as the H3PO4 and silicon concentration and the bath temperature, at constant levels so as to produce constant etch rates and etch selectivities, and process repeatability. As bath life increases, however, these parameters may undesirably vary. It can be seen that it is critical to maintain and control the silicon, Si, concentration in order to maintain constant silicon nitride and silicon oxide etch rates and silicon nitride/oxide etch selectivity. Literature indicates that the oxide precipitate, i.e., the dehydration of Si3O2 (OH)8 to form SiO2 and water, occurs after reaching saturation solubility at about 120 ppm at 165° C. Different temperatures have other saturation solubility levels. The generation of oxide precipitates results in a particle source which is the major yield killer in semiconductor processing.
One known procedure for maintaining a wet silicon nitride etch process is to allow the silica to precipitate and to remove the precipitate by decreasing the temperature. One silica extraction system is designed to remove the generated silica using lower temperature H3PO4 with a high Si concentration. Shortcomings of this procedure include the difficulty in maintaining a high extraction efficiency for precipitated silica, which may melt into the H3PO4 solution again if not removed quickly and if allowed a long reaction time.
It would therefore be desirable to maintain the etching bath to provide relatively constant etching characteristics without suffering from the aforementioned shortcomings.